what would give best results.
A) Slave the 1600mm balscrew steppers or;
(i am not up to speed on how this actually works but i do get the general idea)
B) Connect the ballscrews through a belt and a single stepper.
And is then still a nema 23 3nm ?
Well that question has been debated a lot on the forum and my answer would be non generally is better than the other.

Option B You would possibly need a nema 34 with a high voltage drive like 230V and also you are guaranteed never to rack the gantry. You would also need idler pulleys on the belt.

Option A. you use two drives similar to AM882 (they have stall detection built in) you also need two homing switches so that the controller can make sure the gantry is squared with the homing routine.

The screws in my opinion are best driven by short belts.

Both systems are used widely on the forum.